The proteins in whey are divided into two principal groups, i.e. the globulin fraction containing mainly .beta.-lactoglobulin (.beta.-Lg) and immuno-globulins (Ig-G), and the albumin fraction including .alpha.-lactalbumin (.alpha.-La) and blood serum albumin (BSA). .beta.-Lg is a characteristic protein in milk of ruminants, but does not occur in human milk. .alpha.-La is found in the milk of all mammals and represents a major protein in human milk. This protein is largely used both in preparations of humanized milk and compositions of non-allergenic milk products for infants which are allergic to .beta.-Lg of cow's milk. .alpha.-La represents about 25% of the whey proteins in bovine milk, whereas this contribution in human milk amounts over 40%.
Numerous processes have been proposed for the preparation of .alpha.-La or .beta.-Lg enriched fractions from milk or whey, according to various techniques. These techniques may be split-up in three main categories viz:
1) Selective precipitation of .alpha.-La. PA1 2) Specific separation of .beta.-Lg. PA1 3) Selective fractionation of .alpha.-La and .beta.-Lg.
As mentioned hereinabove the process according to the present invention is of the latter category.
Selective fractionation processes are based on relatively simple physical separation techniques which do not change the natural properties of the whey components. Major advantages of these techniques are the relatively easy scale-up to industrial processes, and the fact that said techniques usually result in valuable by-products with a minimum of waste streams.
The selective fractionation processes are mainly based on membrane separation, using membranes which allow the native .alpha.-La molecules to pass through into the permeate.
U.S. Pat. No. 4,711,953 describes a process using UF-membranes with a cut-off of 50,000 giving an .alpha.-La/.beta.-Lg ratio of 3/2 in the permeate. It is known in the art that this ratio may be increased by using previously defatted whey, obtained by micro-filtration, as described in e.g. WO/89 11 226.
EP-A 0 311 283 teaches the use of UF-membranes, having a cut-off of 100,000, for achieving an .alpha.-La/.beta.-Lg ratio of at the most 3/1 in the permeate.
NL-A-9102003 describes the use of microfiltration for skimmilk after a preheat treatment at 75.degree.-85.degree. C. for 15 minutes. This results in an .alpha.-La/.beta.-Lg ratio of upto 2.7 in the permeate.
All these prior art techniques have the advantage of keeping the whey proteins in their native (globular) state, which facilitates their separation through membranes on the basis of different molecular sizes. It is, however, disadvantageous, that the separation characteristics of the membranes may change during a filtration process as a consequence of membrane fouling. Moreover, slightly damaged or aggregated .alpha.-La molecules in whey are rejected by membranes, giving a reduced yield of .alpha.-La-enriched whey protein permeate.